1. Field of the Invention
The present invention relates to a stripline filter which is used, for example, in a bandpass filter.
2. Description of the Prior Art
A prior art stripline filter is shown in FIG. 1, which is a five stage interdigitated type. In FIG. 1, a reference number 20 designates a dielectric substrate of a rectangular shape having top and bottom flat faces. A bottom flat face (can not be seen in FIG. 1) is deposited entirely with a ground electrode 21. Opposite sides of ground electrode 21 extends around substrate 20 at opposite sides thereof and are connected to side electrodes 21a and 21b which are located at peripheral and opposite side edge portions of the top flat face.
From side electrode 21a, three resonance electrodes 22, 23 and 24 extend parallel to each other towards opposite side electrode 21b. Similarly, from side electrodes 21b, two resonance electrodes 27 and 28 extend parallel to each other towards opposite side electrode 21a in an interdigitated manner with three resonance electrodes 22, 23 and 24. Resonance electrodes 22 and 24 located at opposite ends are provided with terminal electrodes 25 and 26 for the external connection. It is to be noted that each of resonance electrodes 22, 23, 24, 27 and 28 has a short-circuit end connected to the side electrode and open-circuit end spaced away from the side electrode.
Another prior art stripline filter is shown in FIG. 4, which is a five stage combline type. When compared with the filter shown in FIG. 1, the filter shown in FIG. 4 has five resonance electrodes 22, 23, 24, 27 and 28, which extend parallel to each other from side electrode 21a. Also, there is no side electrode 21b.
Stripline filters such as described above are widely used in bandpass filters. As the number of the stripline filters employed increases, the attenuation characteristic of the bandpass filter is improved. Therefore, the number of the stripline filters employed in one bandpass filter has been increased, but resulting in bulky in size and requires a large space to align the stripline filters. Furthermore, when the stripline filters are employed in a microwave filter, a problem arises that the electromagnetic wave leaks outside more than an amount which can be disregarded.